Diesel engines have rather recently begun to gain greater acceptance for use in light duty and passenger transportation applications. In particular, turbocharged diesels have been widely used in Europe for many years. Due to the increased availability of low-sulfur diesel fuel in North America in recent years, such engines are also becoming increasingly available in the United States and Canadian markets as well.
In general, a turbocharged engine (either diesel, other compression ignited, or spark ignited) can produce higher power outputs, lower emissions levels, and improved efficiency than a similarly sized (e.g. in terms of volumetric displacement), naturally aspirated engine. The power and efficiency improvements generally result from forcing of more air, and proportionately more fuel, into the combustion chamber or chambers of an engine than atmospheric pressure alone can achieve. The amount by which the intake manifold pressure of a turbocharged engine exceeds atmospheric pressure is generally referred to as a level of boost and is representative of the extra air pressure in the intake manifold of the engine relative to the pressure achievable without the use of forced induction.
The maximum pressure within a combustion chamber of an internal combustion engine generally limits the power density achievable by the engine. As used herein, the term “combustion chamber” generally refers to a volume existing within a cylinder of such an engine when it is at its minimum volume and being bounded by one or more of the walls of the cylinder, valve surfaces, piston crowns, and the like.
A compression ratio of an internal combustion engine represents the ratio of the volume at its largest capacity relative to its smallest capacity. The effective expansion ratio is the ratio of the cylinder volume at the completion of combustion to the opening of the exhaust valve. The effective compression ratio is the ratio of the volume of the cylinder when the intake valve closes to the minimum volume. In an opposed piston engine, the compression ratio is generally calculated as the cylinder volume when the piston crowns are furthest apart divided by the combustion chamber volume when the piston crowns are closest together. Some engines have effective compression and expansion ratios that differ from one another. As used herein, the effective compression ratio is the quotient of the cylinder volume at the beginning of the compression stroke divided by the combustion chamber volume at the end of the compression stroke, and the geometric expansion ratio is the quotient of the cylinder volume at the end of the expansion stroke divided by the combustion chamber volume at the beginning of the expansion stroke. The effective compression ratio and effective expansion ratio of an engine need not be equal. For example, if the inlet valve closes late so that the piston is already on its way up, the effective compression ratio is less than the geometric compression ratio. If combustion takes place well after top dead center, the effective expansion ratio is less than the geometric expansion ratio. Also, depending on the timing of the closing and the opening of the intake and exhaust valves, respectively, the combustion chamber volume at the beginning of the compression stroke and the combustion chamber volume at the end of the expansion stroke need not be the cylinder volume at a piston bottom dead center position.
For a diesel engine to run reliably, the compression ratio within the combustion chamber is generally maintained above a threshold necessary to create sufficiently high temperatures in the combustion chamber to cause auto-ignition of diesel fuel delivered within the combustion chamber and mixed with compressed gases to form a combustion mixture. A combustion mixture as used herein refers to gases and optionally aerosol droplets (e.g. of fuel delivered by an injector) within the combustion chamber. The gases can include air, vaporized fuel, and possibly other gases (e.g. recirculated exhaust gases, other diluents, etc.).